The present invention relates to a driving device of a stepping motor for improving the operating stability of the stepping motor and for eliminating mis-detection of a driver/detector which detects whether or not the motor rotor has rotated.
FIG. 1(A) shows a structure for a conventional stepping motor. Reference numeral 1 denotes a stator, 2 denotes a rotor consisting of a magnet magnetized in the diametrical direction and 3 denotes a coil.
FIG. 1(B) shows a normal driving pulse 4 which is used to drive the stepping motor. The pulse is applied across the coil 3. The pulse 4 is an alternating pulse having a pulse width of 7.8 msec which guarantees a sufficient output torque for rotating the rotor. Namely the stepping motor does not stop rotating even if the load applied thereto varies and increases.
FIG. 1(C) shows an improved driving method over that shown in FIG. 1(B) and shows what is called a correction driving method. This is an automatic control method which produces a driving pulse width in accordance with the amount of load applied to the stepping motor to thereby reduce the overall current consumption. Reference numerals 5a and 5b are normal driving pulses whose pulse widths are automatically changed in the range of 2.4 msec to 3.9 msec in response to changes in the motor load or the supply voltage. Reference numeral 7 denotes an interval to detect rotation of the rotor 2. A detector (not shown) detects whether the rotor has rotated or not by detecting a voltage induced across the coil 3 by the free oscillation of the rotor 2 after the stepping motor is driven by the normal driving pulse 5a. Reference numeral 6 denotes a correction pulse produced only when the rotor 2 has not rotated by the normal driving pulses 5a and 5b. The correction pulse width has a pulse width of 7.8 msec, which is sufficient to rotate the stepping motor under any load.
FIG. 1(D) shows current waveforms observed in the coil of the stepping motor. An interval "a" denotes a driving pulse width, and an interval "b" denotes a current flowing interval by the voltage induced by the free oscillation of the rotor after the stepping motor is driven by the driving pulse a. The reference character b.sub.1 denotes a waveform in the case the rotor has rotated by the driving pulse "a", and b.sub.2 denotes a waveform in the case the rotor has not rotated. The rotor rotation or non-rotation is detected at an interval "c" during which the characteristic features of both the waveforms b.sub.1 and b.sub.2 are identified. For easy detection, the induced voltage is amplified.
The above-mentioned correction driving method and the amplification of the induced voltage are disclosed in U.S. Pat. No. 4,326,278 in detail.
FIG. 2 shows an embodiment of the driver/detector circuitry which is conventionally utilized. The structure of the detector will only be briefly illustrated as it is illustrated in detail in U.S. Pat. No. 4,326,278. Terminals Out 1 and Out 2 are connected to the coil 3. A voltage induced by the free oscillation of the rotor appears at resistors 105 or 106 after the application of the drive pulse. The voltage is fed to comparators 109 or 110 to be compared with a reference voltage determined by resistors 112 and 113. A voltage higher than the reference voltage is present at the resistors 105 or 106 when the rotor has rotated, so that an output "H" is present at a detection output terminal 121. While there is no problem when a normal driving pulse width is short, the induced voltage is lowered when the normal driving pulse width is prolonged to some extent. For example, when a driving pulse having a pulse width of more than 4 msec in width is applied to the stepping motor which can be driven by a pulse around 2.4 msec in width, the detection voltage is gradually lowered and becomes lower than the reference voltage determined by the resistors 112 and 113. As a result, the detector judges that the stepping motor is not rotating although it actually is rotating. Namely, as the driving pulse width is prolonged, the stepping motor rotates to the rest position during the period while the driving pulse is applied, whereby the free oscillation of the rotor after the application of the driving pulse tends to be reduced and the induced voltage is lowered. Therefore a pulse of more than 4 msec in width could not be used conventionally in the correction driving method. Accordingly, it is necessary to drive the stepping motor by the correction pulse 7.8 msec in width when it has not rotated by the normal driving pulse 4 msec in width, and as a result, it has been difficult to drive the stepping motor at low power consumption.
The present invention aims to eliminate the above noted drawbacks, and therefore it is an object of the present invention to provide a driving device able to cope with a wide range of driving pulse widths.
Although the driving device is not suitable for high-speed rotating instruments, it is effective when low-power operation is required under the conditions that the step rotation interval is several tens msec and the load variation is large. Further, it is most suitable for use in electronic watches since the particular detection elements are not needed and the whole circuits are composed of the C-MOS IC. The embodiment of the present invention will be illustrated taking the application for watches by way of exapmle.
A stepping motor according to an embodiment of the present invention comprises a coil 3 having 8000 turns of wire of diameter 20.mu., a rotor 2 with a diameter of 1.3 mm made of samarium cobalt, and a stator 1 made of 78 permalloy. A power source comprises a silver cell of 1.57 volts. A reference signal source is a quartz crystal of 32768 Hz, and an oscillating circuit, a control circuit and the like are incorporated into a single C-MOS IC chip.